You are what you believe.
At the core of each of us is our belief system. It is around that belief system that a large part of our personal identity is formed. One of the real strengths of fundamentalism is that it provides a stable core belief system. To borrow from 80's new wave and avant-garde band, Talking Heads, “Same as it ever was. Same as it ever was.”
It is much easier to believe you understand who you are and to be stable when your core belief system is stable. For folks like liberals and progressives this is a little more difficult because the walls around our core beliefs are a little less rigid and more willing to flex as new information presents itself. Which means that we, more frequently than fundamentalists, are reshaping our understanding of who we are and how we relate to society, even if in small ways.